A superconducting magnet used in the systems such as a Magnetic Resonance Imaging system (MRI) works at a cryogenic temperature of several tens of K or lower. In general, this cryogenic ambient condition is generated by regenerative cryocoolers such as the Gifford-McMahon (GM) cryocoolers. In the regenerative cryocoolers, appropriate regenerator materials are used in consideration of their temperature dependences of specific heat. Currently, commonly used Gifford-McMahon coolers comprise materials of Cu, Pb and rare earth-based compounds such as HoCu2 or Er3Ni as the regenerator materials working effectively for a temperature range from room temperature to about 100 K, for a temperature range from about 100 K to 10 K, and for a cryogenic temperature range below 10 K, respectively.
In recent years, Pb has been considered to be a material which has a high burden on the environment, and a regenerator material which does not contain Pb is required. Before now, materials such as Bi, Sn and their compounds were proposed.
However, there has been a problem that the materials such as Bi, Sn and the compounds containing them as main components have insufficient specific heat at low temperatures, and thus the cooling properties of the cryocoolers using regenerators filled with such materials as the regenerator materials are inferior to those of cryocoolers using Pb.
In the regenerative cryocoolers such as Gifford-McMahon cryocoolers, pulse tube cryocoolers and Stirling coolers, high-pressure working gas is made flow reciprocally through the void in the regenerator filled with regenerator materials. Furthermore, in the case of Gifford-McMahon cryocoolers and Stirling cryocoolers, regenerators themselves filled with regenerator materials also move reciprocally. Therefore, regenerator materials are required to have mechanical strength against them.